In an assembled electrical circuit, dielectric failure can lead to either electrical or physical breakdown of the circuit. Both materials vendors and circuit manufacturers rigorously test components under fresh and accelerated conditions to decrease the probability of this occurrence. A basic type of electrical failure is the loss of adhesion of the dielectric composition to the underlying substrate. Without acceptable adhesion, the material is not suitable for reliable use.
Adhesive loss is a difficult problem since it is intimately related to the substrate. The problem is compounded by the large number of available substrates. While polyester films are the most widely used in touch switches, polycarbonate and polyimide films are also used. Each film manufacturer typically offers several grades of each product, with different surface characteristics due to variable processing techniques and/or surface pretreatments. The films may also be given a heat treatment to reduce shrinkage in later curing steps.
The common substrate for printed silver and dielectric compositions in touch screen displays and thin-film photovoltaic cells is typically indium tin oxide (ITO) deposited on top of a polyester or glass substrate. A common method for deposition of the ITO is sputtering. Other metal oxides such as zinc oxide may be used in place of ITO although ITO is the most widely used metal oxide of its type. ITO is used quite often because of its ability to function as an optically clear conductive phase. However, ITO has a very low surface energy. Thus, it is very difficult to have printed compositions adhere to the ITO surface.
In practice, most manufacturers first select a conductive ink, and then look for a compatible dielectric. The selection is especially critical in touch screen display and thin-film photovoltaic cell applications since the dielectric is used both to insulate the conductor and to encapsulate it to prevent environmental damage. Lack of adequate adhesion of the dielectric to the substrate and/or to the conductive ink has resulted in limited market penetration for many dielectric compositions, especially those which are ultraviolet (UV) curable.
Existing manufacturing processes dictate that the dielectric be screen-printable and either thermally curable or UV light curable. Faster cures can be obtained with the UV curing and the wide availability of UV curing units makes this a more cost-effective and practical route. The dielectric composition must be compatible with the conductive ink and must meet certain other performance standards. It must cure to a flexible, abrasion-resistant film, with good adhesion to the substrate and to the conductive ink. Crossover applications also require that the conductive ink have good adhesion to the dielectric and, frequently, good adhesion of the dielectric to itself. Electrical requirements call for a low dielectric constant, high insulation resistance and high breakdown voltage. The physical and electrical properties must not degrade under a variety of environmental conditions.
The most widely accepted criterion for measuring the adhesion of polymer thick film materials is the tape test described by ASTM D3359-78, Method B. For films under 5 mils thickness, it requires that a 10×10 grid 30 pattern be made with a sharp cutting instrument through the cured ink to the surface of the substrate. A device for this purpose is available from the Gardner/Neotec Instrument Division of Pacific Scientific. A pressure-sensitive tape, such as 3M Scotch® Brand 600, is applied over the grid pattern and then removed with a continuous, nonjerking motion. Depending on the extent of ink removal, the adhesion is rated from OB to 5 B, the highest rating representing no ink removal.
Many of the inks which fail this Crosshatch test nevertheless exhibit acceptable adhesion in a simple tape pull test. This implies that adhesion loss is due to delamination of the ink from the substrate due to the excess energy imparted to the ink during the cutting operation. Unless this energy can be stopped from traveling laterally across the ink substrate interface, these inks will give poor Crosshatch adhesion. It is frequently observed that inks with nominal Crosshatch adhesion pass or fail depending on the type of cutting pattern; few cuts widely spaced impart less energy than several cuts close together on the same unit area. The ASTM test described above is designed to make Crosshatch testing more reproducible by quantifying the transverse forces applied in any particular situation.
To survive the stress of crosshatching, polymeric inks need to be toughened so that the applied forces are absorbed or dissipated in the vicinity of the cuts and are thus prevented from traveling to the ink substrate interface. One way of doing this is to increase the degree of crosslinking. This technique can be counterproductive in that the resulting composition may become too brittle for a touch panel ink. Another method is to use filler particles such as talc. While this improves adhesion to some extent, it does not provide enough adhesion for an ITO surface. There is a continuing need for a screen-printable, UV curable dielectric composition that provides good adhesion to ITO substrate surfaces.